A large part of our research program is preparation of neoglycoconjugates from synthetic fragments of complex polysaccharides that are part of Gram negative bacteria, e.g. O-specific polysaccharides (O-PS), and evaluation of their immunogenicity. Before we select structures to become antigenic components of these immunogens we have to identify immunologically dominant epitopes of the O-PS. In order to do so, we chemically synthesize oligosaccharides that mimic the structure of the O-PS and study binding thereof with homologous antibodies. To effect conjugation of antigenic oligosaccharides to suitable protein carriers, we prepare them in the form of glycosides whose aglycons make them suitable for conjugation. These materials are then linked variously to suitable carriers, to obtain conjugates which differ in architectonic details. We then evaluate the immunogenicity of neoglycoconjugates thus obtained using small animals as hosts. In continuation of our work on mapping the combining sites of some monoclonal antibodies for Vibrio cholerae O:1, we have synthesized three of the four theoretically possible specifically monofluorinated analogs of the terminal monosaccharide determinant of the O-PS of the Ogawa strain. Work on remaining fourth derivative is in progress. Binding studies with these substances will provide additional information on binding on the molecular level. These results will be complementary to our previous studies that employed synthetic fragments of the O-PS and some related, specifically deoxygenated analogs. In addition, we have developed a new approach to the synthesis of spacer-equipped oligosaccharide fragments of the O-PS of Vibrio cholerae O:1. Work on linking some of these antigens to carrier proteins, to obtain a series of neoglycoconjugates, is in progress. These materials will differ in architectonic details, and will allow rationalization of the effect of certain characteristics, such as the type of linker, linking chemistry or the carrier protein, on immunogenicity. The terminal mono and the disaccharide determinants of the O-PS of Vibrio cholerae O:1, serotype Ogawa, has been crystallized in complex with the protective monoclonal anti Vibrio cholerae O:1, serotype Ogawa antibody, and the structure of the complex has been determined by X-ray crystallography. The found structure fully confirmed our previous findings on the mode of binding obtained by solution studies, and explains the serotype specificity of anti-Ogawa antibodies. Further, it provides a rational basis towards the development of a synthetic carbohydrate-based anti-cholera vaccine.